Nitride semiconductors are excellent candidates as useful materials for short-wavelength light emitting devices because of their wide band gap. Among these, extensive research has been conducted on gallium nitride-based compound semiconductors (GaN, AlGaN, GaInN, AlGaInN, and like GaN-based semiconductors), and both blue and green light emitting diodes (LEDs) have already been put to practical use. Furthermore, a semiconductor laser with oscillation wavelength in the 400-nm band is in strong demand to increase the storage capacity of optical disc apparatuses. For this reason, semiconductor lasers using GaN-based semiconductors have attracted widespread attention, and are now approaching a level of practical use.
It is preferable that a nitride semiconductor bulk substrate be used as a substrate on which the nitride semiconductor crystal is grown. However, because nitride semiconductor bulk substrates are expensive and difficult to obtain, sapphire substrates, SiC substrates, etc., are generally used. However, since these substrates have a lattice mismatch with nitride semiconductor crystals, a GaN layer formed on, for example, a sapphire substrate has a dislocation density of approximately 1×109 cm−2. Such dislocations lower the reliability of fabricated semiconductor devices, and therefore ways to reduce the dislocation density have been studied. Specifically, a method for reducing dislocation density by growing crystals in such a manner that cavities are formed between the substrate and the nitride semiconductor layer is known.
For example, Japanese Unexamined Patent Publication No. 2001-274093 discloses a method for forming a semiconductor substrate wherein peaks and trenches are formed on the surface of the substrate on which the crystals are to be grown, and the crystals of a GaN-based compound semiconductor, etc., are grown from the upper portions of the peaks in a lateral direction, the trenches covered by the semiconductor layer thereby forming cavities.
Japanese Unexamined Patent Publication No. 2001-168042 discloses a method for forming a semiconductor substrate wherein the trenches covered by a semiconductor layer form cavities by providing a mask formed from SiO2, etc., on the trenches of a substrate having peaks and trenches on the crystal growth surface, and growing crystals in a lateral direction with the upper portion of the peaks as the starting point.
Japanese Unexamined Patent Publication No. 2002-9004 discloses a method for forming a semiconductor substrate wherein trenches covered by a semiconductor layer become cavities by providing striped peaks on a GaN layer formed on the substrate, forming masks on the bottom and side surfaces of the trenches, and growing crystals in a lateral direction from the upper portions of the peaks.
Specifically, as shown in FIG. 10, a GaN buffer layer (not shown) is first deposited on a sapphire substrate 1001 at 500° C. while supplying TMG and NH3, the substrate temperature is then raised to 1020° C., TMG and NH3 are supplied, and a GaN layer 1002 is then deposited. Second, as shown in FIG. 11, a striped resist 1003 is formed on the surface of the GaN layer 1002 by photolithography, and the surface of the GaN layer 1002 is recessed (trenches) by dry etching using the resist 1003 as a mask. As shown in FIG. 12, by ECR sputtering, a dielectric 1004 formed from a silicon nitride film is then deposited over the entire surface of the substrate 1001. As shown in FIG. 13, the resist 1003 and the dielectric 1004 above the resist 1003 is then lifted off. Thereafter, a metal-organic vapor phase epitaxy (MOVPE) technique is employed to conduct selective lateral growth of GaN using the exposed portions of the upper surfaces of the peaks of the GaN layer 1002 as seed crystals. The adjacent GaN crystals that thus grow in the lateral direction become united and a GaN layer 1005, which is a flat, continuous film, as shown in FIG. 14, is formed on the surface of the substrate. As a result, cavities 1006 are formed below the GaN layer 1005.
The methods for forming semiconductor substrates disclosed in the above publications are such that trenches are turned into cavities by growing nitride semiconductor crystals with their starting points in the vicinity of the upper surfaces of the peaks on the substrate; however, in cases where a mask is not provided on the surface of the substrate, as disclosed in Japanese Unexamined Patent Publication No. 2001-274093, or where a mask is provided only on the bottom of the trench, as disclosed in Japanese Unexamined Patent Publication No. 2001-168042, cavities may not be formed below the semiconductor layer.
Japanese Unexamined Patent Publication No. 2002-9004 discloses a method for forming a semiconductor substrate in which masks are formed both on the bottom and side surfaces of trenches in a substrate, thereby making the formation of cavities easier than in conventional methods for forming a semiconductor substrate. However, because the formation of masks is conducted by sputtering, the step coverage of trenches and peaks is not good, and the coverage of the side surfaces of the trenches in the substrate tends to be unsatisfactory. Therefore, there is room for further improving cavity formation between the substrate and the semiconductor layer. Furthermore, because the resist pattern on the upper surfaces of the peaks and nitride silicon film are lifted off, it is difficult to remove the resist pattern on the upper surfaces of the peaks in an even manner, and some of the resist patterns remaining on the upper surfaces of the peaks may prevent crystals from growing.